The present invention relates to surgical devices for stabilizing and/or fusing adjacent bone structures, and, more particularly, to surgical devices for stabilizing and/or fusing the spine and for implantation between the vertebrae, including the intradiscal space. Generally, this invention concerns internal fixation devices, particularly spinal fusion and related implants.
Spinal degenerative diseases (e.g., stenosis, disc disease, spondylosis, etc.) trauma, aging, or a herniated disc can cause compression in the spine thus applying pressure to the nerve roots and/or spinal cord. The compression produces progressive pain, loss of movement and sensation, and sometimes, permanent disability. Spinal fusion is among the standards of care for surgical decompression and stabilization of the spine. Fusion, known also as arthrodesis, is accomplished by the formation of an osseous bridge between adjacent motion segments. The goals of spinal surgery include relieving spinal cord/nerve compression, promoting spinal fusion, increasing stability, maintaining spinal alignment, and restoring disc height. Ideally, reconstructive surgery would result in total spinal fusion with an excellent clinical outcome.
For over 40 years, removal of the problematic disc and fusion of the adjacent vertebrae has been the common treatment for degenerative diseases. The classical surgical procedure is discectomy and interbody fusion with an iliac crest autograft with or without internal fixation. A discectomy typically requires the removal of a portion or the entire intervertebral disc. Different types of grafts (e.g., autograft, allograft, or synthetic ceramics) are used to fill the disc space.
Unfortunately, the use of bone grafts presents several disadvantages. Autogenous bone, which contains matrix molecules and living cells such as osteoblasts that facilitate fusion, is the ideal bone graft; however, postoperative pain is often greater at the harvest site than the surgical site. Additionally, autografts removed from a patient may not yield a sufficient quantity of graft material. Harvesting bone is also associated with high rates of harvest site morbidity and can increase the risk of infection and blood loss. Alternatively, allografts obviate the need for bone harvesting, but have inconsistent mechanical properties. Allografts can also transmit diseases or cause infections, and they have unpredictable and slow fusion rates. Autografts and allografts alone may not provide the stability required to withstand spinal loads and are subject to collapse or failure due to a lack of strength.
In the mid-1970's, Bagby found the clinical results of harvest site morbidity to be unacceptable. In U.S. Pat. No. 4,501,269, he describes the “Bone or Bagby Basket” to eliminate bone graft harvesting and promote bone fusion. Due to the drawbacks of traditional fusion techniques, his initial invention was important and innovative, and it has continually been improved in both design and material selection. These interbody fusion devices are designed to stabilize the vertebral bodies, hold osteogenic material, and promote early stabilization and fusion. The rigidity and structural design of the devices must be able to support the axial loads in the spine. Commercially available spinal interbody fusion devices are made of stainless steel, titanium alloy, carbon fiber, or allograft bone. Often, these devices have void spaces or perforations to allow bone ingrowth.
While carbon fiber and metal interbody fusion devices offer strength advantages, they have several disadvantages. Metal interbody fusion devices are a permanent foreign body and are difficult to remove during revision surgery. Due to the difference in mechanical properties of bone and metal, the main concern of metal interbody fusion devices is stress-shielding, which may cause bone resorption or osteopenia. Although these devices have demonstrated an ability to facilitate fusion, a sufficient fusion is not always achieved between the bone grafts housed within the cage and the vertebral endplates. Achieving a complete bony union in the middle portion of the cage has been particularly problematic. Clinical fusion outcomes may be difficult to assess with metallic interbody fusion devices due to the artifacts and scattering during postoperative CT or MRI scans. Often a complete bony union cannot be seen, making fusion results unreliable. Carbon fiber cages are radiolucent and have properties, such as modulus of elasticity, similar to bone; however, they are also a permanent foreign body. Long-term results with metal and carbon fiber interbody fusion devices are unknown due to the relatively recent development of the implants. Metal cages have been known to fatigue and will eventually fail if a solid bony fusion is not achieved. Over time, metal and carbon fiber cages may migrate or have significant subsidence into the vertebral bodies.
Gjunter (U.S. Pat. No. 5,986,169) describes a porous (i.e., 8 to 90% porosity) material made of a nickel-titanium alloy. The pores form a network of interconnected passageways that permit fluid migration through the material. The material may be used for biomedical implants or non-medical applications. Kaplan (U.S. Pat. No. 5,282,861) and Zdeblick et al. (U.S. Pat. No. 6,613,091) discuss a similar porous material made of a carbon-tantalum composite that could be used to create an implant device. The elasticity of the porous materials is similar to live bony tissue; however, most of the disadvantages associated with carbon fiber and solid metal internal fixation devices still apply to the porous nickel-titanium and carbon-tantalum alloy materials. For example, the porous metal implants remain permanently implanted in the body.
To avoid the disadvantages of metal and carbon fibers devices, bioresorbable materials have been used for years as sutures, bone plates, screws, pins, and other medical devices. A few advantages of bioresorbable implants include biocompatibility, predictable degradation, and complete resorption via natural pathways by the body over a period of time. Polymers are advantageous over other bioresorbable materials, such as ceramics, because they have high toughness and are highly reproducible. The toughness significantly reduces the danger of polymers failing by brittle fracture. Bioresorbable polymers can be formed into spacers, wedges, threaded cages, and a variety of other shapes (e.g., spinal interbody fusion devices).
Bioresorbable implants are transparent to x-rays, and therefore allow, for example, postoperative clinical assessment of a bony union, thereby overcoming one disadvantage of metallic implants. They can perform all the requirements of an interbody cage by providing immediate stability, maintaining foraminal distraction, restoring disc height, and allowing bone ingrowth and fusion. Bioresorbable interbody fusion devices can be produced that provide sufficient strength retention (up to 12 months or longer) in order to allow fusion to occur, then resorb after they are no longer needed. They have the compressive strength to withstand and carry the spinal axial loads; however, they have a modulus of elasticity similar to bone, which limits stress-shielding. Bioresorbable implant devices may feature or contain osteogenic material to attract bone and cells to the implant. Additionally, the bioresorbable devices may be hydrophilic and/or porous. Porous, hydrophilic devices promote the migration of fluid material into the implant, thus allowing a wide variety of tissue ingrowth. The porous bioresorbable implants are fully capable of being replaced by the patient's own bone growth.
Lynch (U.S. Pat. No. 5,306,303), McKay (U.S. Pat. No. 6,346,123) and Webb (U.S. Pat. No. 6,503,279) all describe bioresorbable, porous ceramic materials that may be used in medical implants. McKay and Webb specifically describe an intervertebral fusion device. Due to the brittle nature of ceramic materials, particularly as degradation occurs, the disclosed materials may not withstand the axial loads or cyclic loading of the implant site (e.g., spine) without fracture, collapse, and ultimately device failure.
McKay (U.S. Pat. Nos. 5,702,449 and 6,039,762) describes a spinal cage with an inner core of porous biocompatible material, preferably porous ceramic, which allows tissue ingrowth, and an outer body that can withstand compressive loads. The porous biocompatible material may protrude from the outer shell to permit contact with the vertebral bodies. The implant design with the resorbable inner core does not allow for the use of a bone graft within the device. A high strength outer shell may provide sufficient support; however, it brings concomitant property mismatch with natural bone. Bioceramics as used to form the outer shell are brittle and may fracture under high spinal loads.
Moumene and Serhan (U.S. Pat. No. 6,569,201) disclose a fusion cage with a structural bioresorbable layer disposed upon the outer surface of a non-resorbable support. The purpose of the non-resorbable support is to act as a scaffold for the bioresorbable layer and to hold a bone graft or osteogenic material. The bioresorbable layer would resorb over time, gradually increasing the loading on the bone graft and fusion cage. If the bioresorbable layer and bone graft degrade before fusion can occur, the non-resorbable support may cause stress-shielding. Depending on the thickness of the bioresorbable layer, complete degradation of the layer may cause a great decrease in disc space height. The non-resorbable support will remain a permanent foreign object in the body.
Gresser et al. (U.S. Pat. Nos. 6,241,771 and 6,419,945) describes a spinal interbody fusion device composed of 25-100% bioresorbable material. The device is composed of a resorbable polymer that can produce acidic products upon degradation and includes a neutralization compound to decrease the rate of pH change as the device degrades. In order to withstand the maximum physiologic loading, of at least 10,000 N (the maximum expected lumbar load), the disclosed device must be reinforced with fibers. The device is not porous, consequently limiting bone ingrowth. Similar to metal interbody fusion devices, the device may have void spaces to hold osteogenic materials, such as bone grafts or other osteogenic material. The disclosed device will slowly degrade and lose strength over time with complete resorption predicted to occur by one year. Clinically, complete fusion and bony union may take longer than one year in unstable patients. If fusion of the endplates through the disk space does not occur, the short-term resorption of the device may lead to collapse of the disk space.
Bioresorbable interbody spinal fusion devices offer solutions to disadvantages related to bone grafts and metal and carbon fiber cages. Autografts require bone graft harvesting, which causes postoperative pain and morbidity at the harvest site. Allografts put the patient at risk for infection or transmitted diseases. Metal and carbon fiber cages remain permanent foreign bodies. Metal cages can cause stress-shielding and make fusion assessment difficult. They may also migrate from the implantation site or subside into the vertebral bodies. A need exists for an interbody spinal fusion device that achieves a successful fusion and bony union while avoiding drawbacks associated with the use of metal and carbon fiber devices or bone grafts.